Since World War II, chaff deployment has been one of the most cost-effective electronic countermeasures (ECM) to provide self-protection of tactical aircraft from radar directed weapons. Chaff is relatively inexpensive, and yet has almost universal effectiveness against radars of all types. Conventional chaff basically consists of thin wires or foil strips cut to a length of one-half wavelength (actually, slightly less than that length is optimum) of the radar. That basic design has changed little in the almost half century since its introduction. Radar electronic counter-countermeasures (ECCM) designers have labored to use some means to discriminate the chaff from the aircraft returns. There has been, however, no particular technique which completely negates the effect of chaff, though some techniques can reduce the effects.
Some ECCM designers noted early that chaff fell with a primarily horizontal orientation. Vertically polarized radars would be less effected. The chaff designers, then started to weight some of the chaff dipoles to create a more uniform distribution of polarization returns. When ECCM designers tried to use the half-wavelength resonance of chaff to reduce its effectiveness by deploying frequency agile radars, the chaff designers used a mix of different length chaff to create a broad band of frequency screening. To provide greater shielding capability, chaff designers also improved the cross section per unit volume capability by using thinner chaff dipoles, e.g., coated glass strands. When ECCM designers tried to use the deployment time of the chaff cloud to discriminate it from the aircraft, the chaff designers came up with rapid blooming chaff. Over the years, chaff designers have continued to experiment with a variety of enhancements to chaff. Odd shaped chaff which spins when deployed, creates enhanced doppler returns to help defeat moving target discriminators. Various shapes, coatings (to reduce a tendency of the chaff dipoles to clump together in a "birdnest", and improve blooming), deployments (explosive dispersion, slip-stream dispersion, rocket fired dispersion, etc.), and packagings have been used to extend the effectiveness of this ECM technique.
Radars have also advanced, and one advancement has been to extend radar operation to higher frequencies. Chaff can now be made into very small lengths of very thin strands. These changes extend the utility of standard chaff through the microwave bands, and into the millimeter band. At some point, however, as frequencies are increased, this form of chaff will produce diminishing returns. This thin-wire chaff remains a resonant dipole, which has a peaked response at only one frequency, and some return at multiples of it. It also suffers from loss due to random orientation of the dipoles. Most importantly, dipole chaff in the millimeter region is, because of physical configuration, capable of less effect with increasing frequencies.
Decreasing effectiveness occurs because chaff strands are already being made as thin as possible, even in the microwave band. Because thickness does not significantly improve chaff performance (as long as sufficient conductive volume in the coating is present), the thinnest chaff possible produces the greatest equivalent radar cross section (RCS) for a given volume of chaff package. At some point, however, the coated glass fibers will reach some practical minimum thickness. When this point is reached, the effectiveness of the chaff will degrade with increasing frequency.
An individual average (i.e., average over all possible orientations) chaff dipole has a return roughly equal to .rho..sub.1 =0.15.lambda..sup.2, where .lambda. is the wavelength. The equivalent radar cross section of a chaff package is equal to a constant times the cross section of one dipole, times the number of dipoles in the chaff package (i.e., .rho.=C.sub.1 .rho..sub.1 n, where C.sub.1 is a constant and n is the number of dipoles. If thickness of the dipoles does not change with increasing frequency, the number of dipoles in a given chaff package volume will increase as a function of 1/.lambda. (i.e., n=C.sub.2 /.lambda., where C.sub.2 is some constant). The net result will be the total radar cross section of the chaff package volume will be a function of .lambda. (i.e., .rho.=C.rho..sub.1 n=C.sub.1 .lambda..sup.2 C.sub.2 /.lambda.=C.lambda., where C is a constant), or the reciprocal of frequency.
As long as the chaff dipoles can continue to be made proportionately thinner as they are shorter (with increasing frequency), it is possible to gain increasing cross section/package as a function of frequency. Once a point of minimum practical thickness has been reached, however, the equivalent cross section/package decreases with frequency.
Current chaff dipoles are reaching a limit of effectiveness in the millimeter band. They have two deficiencies, illustrated by FIGS. 1A and 1B, that will not be deficiencies in the chaff proposed herein. FIG. 1A illustrates the return of a typical chaff dipole with a length-to-width ratio of 112. This would correspond to a dipole of 25 micron thickness that was cut for resonance at about 55 GHz (i.e., about 0.56 cm long). It shows how the RCS decreases sharply after resonance. The bandwidth for this low length-to-width ratio is about 25%. At typical length-to-width ratios of 1,000 or 10,000 the bandwidth would only be about 15% and 10% respectively. To cover a broad range of frequencies a variety of dipole lengths would have to be included in the chaff package volume. This dilutes the effective RCS of a chaff package at any given frequency. FIG. 1B illustrates the RCS which can be packed into a given volume chaff package verses frequency (assuming that minimum practical thinness has been reached). The breakpoint in FIG. 1B occurs when the minimum practical thickness has been reached. The entire curve, however, would also have to be shifted downward if a broadband of frequencies needed to be covered with a single chaff package.
Chaff is also used as a research tool for tracing air motion in the atmosphere. Aircraft seed the atmosphere with radar reflecting chaff so that radars can obtain measurement data on wind direction and speed. Ideally such chaff should be highly radar reflective, have good volume-to-cross section ratio, have low fall rates, and be environmentally benign. Modern aluminized glass fiber chaff possesses most of these qualities, but would have greater utility for tracing atmospheric air motion if it had a lower fall rate.
Aluminized glass fiber chaff is touted to have a fall rate of 0.3 meters/second. This is at sea level; it has a higher fall rate at upper altitudes. Measured data, moreover, often shows faster fall rates on this chaff. The new chaff described herein can be made with fall rates less than 1/10 that of conventional chaff.
Because chaff dipoles, at resonance, have almost the same radar cross section (RCS) regardless of their thickness, they are made as thin as possible. Making the dipole chaff as thin as possible also makes the fall rate as low as possible. This makes the chaff less broadband, but that is not usually an issue for this application. Current aluminized glass fiber chaff has a thickness of about 25 microns. At this thickness, many dipoles can be packed into a small volume at microwave frequencies and lower. To make the chaff thinner yet, would result in reduced conductive volume, and increased breakage of the dipoles. In effect, current dipole chaff has reached limits in its ability to be effective at very high frequencies, and also in its fall rate.